Very loose galactic clusters are called Stellar Associations and were discovered in the late 1940s. These associations are almost spherical in shape and yet have very low spatial density. The differential revolution about the galactic center should have elongated these groups of stars in a few hundred million years. This is due to the fact that the portion of the galactic disc nearer the center of the galaxy revolves more rapidly than the outer edge of the disk, in a pseudo-Keplerian motion. The lack of elongation noted in associations indicates that they must have been recently formed and are expanding at velocities of the order of 5 to 10 km/sec. If so, their ages cannot exceed a few millions or tens of millions of years. Young associations are of spherical shape (circular in outline) and older associations suffer distinct elongation as would be expected from the effect of galactic differential rotation, the spinning galactic disk.

The official definition of stellar associations in the words of their founder V.A. Ambartsumian: "O-Associations are stellar systems where the partial density of '0' and 'B2' stars [spectral class] is larger than the average field density of these stars in such a way that this difference cannot be explained by chance fluctuations; moreover, '0' or 'BO' stars are present."

The radii of these associations range up to 200 parsecs, far exceeding a typical open cluster or even that of the mighty globulars. It has been found that many associations outline the three major spiral arms of our galaxy. An association may contain 100 stars and it is estimated that ten million stellar associations have passed through their evolutionary cycle during the lifetime of our galaxy, each ending in total disintegration and the scattering of its members through the galaxy.

Perhaps the most famous association is that in Orion, where the great nebula and its central cluster form the nucleus of a large expanding association. Many associations include well-known galactic clusters and emission nebulae. Some are so young that only their most massive stars have had time to condense out of the interstellar medium and reach the hydrogen burning stage on the main sequence. The less massive young stars (still in the contracting stage) are usually embedded in bright and dark nebulosity and many of these stars are variable. It has even been reported that a star (FU Orion) has become visible in the Orion association where a few years prior none appeared in the photographs.